Archive for September, 2009

 

Water Rocket Car R/C Development

The last week or so we have been working on a R/C (remote control) addition to our water rocket car. There is much to learn it seems in the R/C world and we have been on a steep learning curve this last week. We assembled all the pieces we needed to get it working – these are

– Transmitter – Ours is a Futaba T2PH 29Mhz AM 2 channel radio transmitter (built in antenna)
– Receiver – A Futaba R122JE – 2 channel 29Mhz receiver with built in antenna
– A battery pack with a R/C connector
– A servo – we used initially a 46g servo which has 3.4kg/cm of torque

futaba transmitter

futaba transmitter

servos receiver and battery pack

servos receiver and battery pack

Initially we just did some testing of the R/C gear and this is really not too difficult to get working, 12 x AA batteries and some wires and we were in business, both servos were responding to commands from the transmitter.

In the research we did, there doesn’t seem to be a lot of talk about the range of these transmitters, so we tested what range we could get out of it. We did some testing by having one person hold the servo / receiver and battery pack and the other hold the transmitter and keep walking then every 10 steps or so turn the transmitter steering to see if the servo’s reacted. My other trusty assistant was keen to help out and would yell back “STILL WORKING” each time the steering was turned on the transmitter. We managed to walk 170m away up a hill and over the other side and it was still working, we didnt have line of sight and there was 2 large trees in the way and it was still working. We are not sure really how far it will go because we stopped the test there as I couldn’t hear the replies any more not that it wasn’t still working. So we are happy that the range of this transmitter will be fine for our present needs.

The steering was custom made – based around a simple 1 steering arm design connected to the servo. The steering arm is made of metal (steel) and can probably be replaced with something lighter like aluminum or carbon fibre but will do for the moment.The steering arm is connected to the servo via a single rod which has ball joints at each end. This allows the rod to stay horizontal throughout the servo’s circular movement

Steering Arm

Steering Arm

Each wheel connects to a wheel hub which pivots on a single connection to the body of the car. The steering arm is connected to the hub with a bolt and nut which is fitted loosely to allow movement. The nut is glued to the bolt so it doesn’t fall off and detach the steering arm

Wheel hub and steering arm attachment

Wheel hub and steering arm attachment

Here is the steering with everything attached. A second level was temporarily installed to hold the receiver and battery pack just for testing

R/C Steering Setup

R/C Steering Setup

Here is a video of the steering working on the bench. This was also our first attempt at a wide screen video … :)

We did some rolling tests of the water rocket car and had some success with it steering when it was rolling. The conclusion was that the servo is just not quite strong enough for our application. We have been doing some research and have a stronger servo in mind which has torque of 12.4kg/cm which should do the trick. Once this is ordered and fitted we will do some more trials and work up to full pressure tests ….

Posted by on September 30th, 2009 2 Comments

More Small Diameter Nozzle Testing

We finished off some more small diameter nozzle testing this week. With some interesting results.All Test were performed with a 1.5L bottle with 600ml of water

We will be able to use results to steadily increase the instantaneous thrust on the water rocket car.

Picture Diameter (mm) Pressure (psi) Time to empty bottle Area of Nozzle

Gardena 9mm Joiner

Gardena 9mm

9 140 0.433 63
9 80 0.533 63

8mm Nozzle Insert

8mm Nozzle Insert

8 140 0.533 50.2
8 80 0.734 50.2

7mm Nozzle Insert

7mm Nozzle Insert

7 140 0.467 38.4
7 80 1.000 38.4

6mm Nozzle Insert

6mm Nozzle Insert

6 140 1.000 28

5 x 2.5mm Nozzle Insert

5 x 2.5mm Nozzle Insert

5 x 2.5 (star) 140 1.066 24.55
6 80 1.243 28
5 x 2.5 (star) 80 1.400 24.55

These nozzle inserts may be of use to vertical water rockets as well

Here is a short video of the 5 star patter test at 140psi. All the others were a little boring so I wont bother posting them, the results are above

Posted by on September 18th, 2009 Comments Off

Water Rocket Car MkVIII – Launch Day

The goals of this launch day were to try to control the thrust of the water rocket car and bring the car home in one piece. We tried to achieve this by

– Adding a rear wing to provide down force to the back of the car
– Reducing the size of the nozzle to 6mm to even out the thrust

Here is the video


First Test – 1500ml water and 80psi

This was just to see if the car was running straight. We found that with the 6mm nozzle insert in and 80psi most of the water actually didn’t come out of the bottle and it just trundled along. That was ok, we made some wheel alignments. Distance was probably 30m and stopped by foot

Second Test – 1500ml water and 80psi
Another low pressure test run. It was tracking pretty straight – distance about 40m stopped by foot. 2 broken screws .. remember to bring more next time

Third Test – 1500ml water and 120psi
This was the first high pressure launch with the 6mm nozzle insert and wing. The thrust was pretty good and most of the water was expelled from the bottle. The thrust was definitely lower than previous 12-psi launches and it lasted longer which is what we expected. We are not sure how much difference the wing made as the car wasn’t traveling that fast. There was a bit of left movement and the car hugged the gutter a little but didn’t hit it. It ended up hitting the gutter at the end of the road pretty hard damaging the front bumper. It could have easily gone another 30-40m if there was road available. Distance ended up being 84m, a new record for us… just :)

Fourth Test – 1500ml water and 90psi
We were really keen to try to get the 100m distance record, so we moved the launcher back 20m. Unfortunately we had an issue with the valve in the launcher quick connect and it basically was only letting 90psi into the bottle, the rest wouldn’t get through and just the air line was pressurizing up to 120psi. This is something we know how to fix, but couldn’t do it on site, so we launched at 90psi anyway. It doesn’t go very fast as a lot of the water is still in the bottles. It did go left but we are pretty sure this was just a steering alignment issue. We were breaking the heads off screws (4 of them) trying to realign the front wheels, so this was as good as we could do. The car glanced the gutter but it did keep rolling and rolling. It eventually stopped at 96m .. so close …not very dramatic .. but a good distance run.

Conclusion
We had basically accomplished what we set out to do and that was control the thrust and rear end of the car. We were probably too conservative on the nozzle and will test higher diameter nozzle inserts to get the speed up again. Good news is that the car came home with all wheels in place and a new distance record for us under the belt. Now to get that 100m record :)

Posted by on September 8th, 2009 Comments Off

Small Diameter Nozzle Testing

Todays testing was to ensure the smaller diameter discs would not crack or break when high pressure water passed through them. We setup a vertical test stand with a 1.5L bottle and a standard water volume of 600ml.

We first tested the bottle with the 6mm diameter disc inside the nozzle at both 80psi and 140psi
The 6mm disc was then removed and tested again at 80psi and 140psi.

The disc survived the testing without any signs of cracking or stretching

It was obvious even in real time that the 6mm diameter disc slowed the water coming out quite significantly. So we slowed the videos down in VirtualDub (video processing software) and recorded the exact time it took to empty the water from the bottles. The results were

Nozzle Diameter  (mm) Pressure  (psi) Time to Empty Bottle (seconds)
6 80 1.243
6 140 1.000
9 80 0.533
9 140 0.433

Here is a video of the testing

The 6mm nozzle disc has a 44 percent smaller area than the 9mm nozzle.The water took 2.3 times longer to be pushed out of the bottle or 43% slower than the 9mm at the same pressures.

Looks like the size of the nozzle discs we make will slow the water down in proportion to the surface area of the hole. This will provide a longer thrust phase with lower magnitude thrust which is what we need to control our thrust steering problem.

Posted by on September 5th, 2009 Comments Off